Planar substrates, such as films, plates, etc., that are made of polymeric materials often have insufficient slip and friction properties during production or processing. This frequently creates a number of undesirable problems which are attributed mainly to blocking when the substrates touch one another or when a substrate touches a guide member or deflecting element during processing.
Several methods have been proposed to overcome the foregoing problems. These include: mixing inert particles into the polymer material; coating the substrate's surfaces with preparations containing inert particles; or texturing the substrate's surfaces by mechanical, chemical or physical processes. The common goal of each of these methods is the formation of peaks on the substrate's surface. These peaks perform a spacing function and should, therefore, lessen and/or eliminate the blocking of the individual layers of the substrate within stacks. Also, gliding of the substrate over rolls and/or contact with guiding or deflecting elements is improved by the peaks on the substrate's surface.
The foregoing methods have resulted, in part, in clearly improved slippage properties for planar substrates. These improvements have caused a favorable effect on the practical value of those substrates. But, in practice, it became evident that these methods, particularly, when used to manufacture high-grade substrates such as oriented films, caused a number of disadvantages. Moreover, these methods are very costly and have limited applicability for procedural reasons. These procedural reasons are discussed hereinafter.
When inert particles are mixed into the polymeric material forming the planar substrate, the following disadvantages arise: random surface textures (or structures); inadequate abrasion resistance that can cause an increase of the substrate's friction coefficient; considerable cloudiness; poor cutability; interference with the electrical properties; etc. These disadvantages can be traced to: the inert particles; the process of preparing and mixing the polymer and inert particles; and the process by which the polymers are manufactured. Particularly, however, the particles cause the disadvantages because they have: an insufficient strength; an affinity for the polymer; or a strong tendency to agglomerate.
Exemplary of the above-discussed surface texturing by mixing inert particles into the polymer is U.S. Pat. No. 4,725,472. Therein the substrate has peaks surrounded by depressions, the peaks being produced by inert particles. This substrate has several disadvantages. They are: nonuniformity of peaks; peak size is largely dependent upon the size and the amount of inert particles; peak size is also influenced by the form and surface distribution of those particles; and it is not generally feasible to produce these substrates.
In European Application No. 153,853, polyethylene terephthalate substrates are improved by the use of a coating preparation that is applied during the manufacturing process. The problems discussed above, in relationship to inert particles mixed into the polymer, are also encountered in this method. In addition, these coating preparations cause a waste disposal problem because films cannot be recycled. The non-recyclability occurs because the components of the coating preparations are not chemically compatible with the polymer.
In WO-A-87/02173, surface textures are produced by a low temperature plasma treatment. By this method, the surface is given improved slippage, as well as, better adhesive properties for subsequently applied layers. U.S. Pat. No. 4,247,496 discloses substrate surface texturing by means of UV-radiation. Both procedures are costly. Moreover, they are integrated in stretching processes and/or connected with subsequent stretching processes.
DE-A-4,015,658 discloses a process for surface texturing oriented thermoplastic films by the use of rollers having a specific textured surface. However, the disclosed process is free of additives aside from the catalyst precipitates which are created during raw material production. The disclosure makes no reference to any specific catalyst precipitate or to the size of the precipitate or to the grain size distribution of the precipitate.